Generally, a crane is equipped with a back stop for restricting an up-down angle of a boom attached to a main body of the crane so as to be capable of going up and down. As recited, for example, in Patent Literature 1, the back stop includes an outer casing having an opening portion with a front end thereof opened, and an inner casing having a front end side slidably fitted into the outer casing from the opening portion. One of a base end portion of the outer casing and a base end portion of the inner casing is turnably connected to a crane main body and the other base end portion is turnably connected to a boom.